With the recent trend of electric appliances toward a less weight and a smaller size, lithium secondary batteries are demanded which enable higher energy density. Further, with spread of the application fields of the lithium secondary batteries, improvements are required for higher characteristics of the batteries.
Generally, the lithium secondary battery employs, a carbon material which is capable of storing and releasing lithium ions as an active material for a negative electrode; a lithium-containing metal oxide such as LiCoO2, LiMn2O4, and LiNiO2 as an active material for a positive electrode; and a solution of a lithium salt dissolved in a mixed solvent composed suitably of a solvent with a high dielectric constant and a low viscosity solvent as the non-aqueous electrolyte solution. In such a lithium secondary battery, in charging, the lithium is released from the active material for a positive electrode and is stored by the active material for a negative electrode, whereas in electric discharge, the lithium is released from the active material for a negative electrode and is stored by the active material for a positive electrode.
The aforementioned solvent with a high dielectric constant includes carbonic acid esters such as ethylene carbonate, and propylene carbonate; and carboxylic acid esters such as γ-butyrolactone. The aforementioned low viscosity solvent includes chain carbonic acid esters such as diethyl carbonate, and dimethyl carbonate; and ethers such as dimethoxyethane. The aforementioned lithium salt includes LiClO4, LiPF6, LiBF4, LiCF3SO3, LiN(CF3SO3)2, and LiN(CF3CF2SO3)2.
When such a lithium secondary battery is overcharged, with progress of the overcharged state, excessive release of lithium will be caused at the positive electrode, whereas excessive storage of lithium will occur at the negative electrode, and in some cases, metallic lithium may deposit. In such a state, both the positive electrode and the negative electrode will become thermally instable to cause decomposition of the electrolyte solution and violent heat generation, thereby causing abnormal heat generation in the battery to cause the problem of impairing the safety of the battery. Such problems become especially significant with the increase of the energy density of the non-aqueous electrolyte solution battery.
To solve the above problems, techniques are disclosed which employ a small amount of an aromatic compound added as an additive to the electrolyte solution of the non-aqueous electrolyte solution battery to ensure the safety of the battery in an overcharged state.
Japanese Patent Laid-Open No. 106835/1997 discloses a method for protecting a battery in an overcharged state by addition of a small amount of biphenyl, 3-R-thiophene (R being a bromine atom, a chlorine atom, or a fluorine atom), furan, or 3-chlorothiophene, and a battery containing such an additive. In this method, at a voltage higher than the maximum working voltage, the additive will polymerize to increase the internal resistance of the battery to secure the safety of the battery in overcharging. However, when biphenyl is used as a additive, because biphenyl is solid, biphenyl is less soluble in the electrolyte solution, and a portion of the additive may precipitate in a low-temperature operation to cause the problem of lowering the battery characteristics. Furan or 3-chlorothiophene is liable to be oxidized to cause the problem of impairing the battery characteristics.
Japanese Patent Laid-Open No. 58116/2000 discloses terphenyl, and alkyl-substituted terphenyls as the additive. Japanese Patent Laid-Open No. 15158/2001 discloses p-terphenyl as the additive. In these disclosures, the additive is used in a small amount to secure the safety of the battery in the overcharge in the similar manner. These additives are also solid, and are less soluble, causing the problem of deterioration of the battery characteristics such as low-temperature characteristics. In particular, because m-terphenyl and p-terphenyl have a higher melting point, and are not sufficiently soluble in the solvent depending upon a kind of organic solvents, it is disadvantageously difficult to practically use them as batteries.
In view of the above problems of lithium secondary batteries, the present invention intends provide a non-aqueous electrolyte solution containing an additive which does not affect adversely to the battery characteristics such as low-temperature characteristics and storing characteristics and functions effectively against overcharging, and a non-aqueous electrolyte solution secondary battery using the same.